Four-channel stereophonic reproducing system

ABSTRACT

A four-channel stereophonic reproducing system for reproducing CD-4 type four-channel records. At the time of start of playback of a four-channel record and upon appearance of the sub-carrier wave of the sub-channel signal, the muting circuit provided in the sub-channel signal processing circuit is not immediately rendered inoperative lest a large d-c level change take place at the output side of the detector resulting in production of a loud foreign sound from the loudspeakers. To achieve this operation the muting circuit is gradually made inoperative after appearance of the sub-carrier wave of the sub-channel signal. Also, a fourchannel record can be reproduced as a two-channel reproduction with passage of the sub-channel signals blocked through operation of a select switch.

United States Patent Sugimoto et al. Oct. 7, 1975 [5 1 FOUR-CHANNEL STEREOPHONIC 3,842,211 10/1974 Metro 179/15 BT REPRODUCING SYSTEM [75] Inventors: Yukio Sugimoto, Katano; Makoto Primary Examiner \l/mcent Canney I Okamasa Kyoto; Toyoshi Attorney, Agent, or Fzrm--Stevens, Davis, Miller &

Fukumura, Neyagawa, all of Japan Mosher [73] Assignee: Matsushita Electric Industrial Co.,

Ltd., Osaka, Japan [57] ABSTRACT [22] Filed: May 20 1974 A four-channel stereophonic reproducing system for reproducing CD-4 type four-channel records. At the PP NOJ 471,827 time of start of playback of a four-channel record and upon appearance of the sub-carrier wave of the sub- [30] Foreign Application Priority Data channel signal, the muting circuit provided in the sub- M 25 973 J 48 61936 channel signal processing circuit is not immediately ay 1973 1937 rendered inoperative lest a large d-c level change take May dpan place at the output side of the detector resulting in production of a loud foreign sound from the loud 5" speakers. To achieve this operation the muting circuit is gradually made inoperative after appearance of the [58] Fleld of Search 179/1001] sub-carrier wave of the sub-channel signal. Also, a

1 four-channel record can be reproduced as a twochannel reproduction with passage of the sub-channel [56] UNnE S r S rENTS signals blocked through operation of a select switch.

3,825,697 7/1974 Cornell 179/15 ET 5 Claims 2 Drawing Figures Isl +B l3 EQUALlZER DETECTOR AMPLIFIER Sheet 1 of2 U.S. Patent Oct. 7,1975

US. Patent Oct. 7,1975 Sheet 2 of 2 3,911,231

FIG. 2

" L l3 5 s -"R6 TR] FM/PM DETECTOR -4' EQUALIZER l4 RI AMPLIFIER SWITCHING CIRCUIT F OUR-CHAN NEIL STEREOPHONIC REPRODUCING SYSTEM This invention relates to four-channel stereophonic reproducing systems for use in reproduction of CD-4 type four-channel records. Its object is to provide a four-channel stereophonic reproducing system which minimizes foreign noise and sounds that tend to be produced when playback of a record is started. The system also permits reproduction of a four-channel record as a two-channel reproduction.

In the general CD-4 type four-channel record, a composite signal obtained from a first main channel signal constituted by a sum combination of first and second audio signals and a first sub-channel signal constituted by a frequency or phase modulation of a difference combination of the first and second audio signals on a predetermined sub-carrier is recorded in one sound groove, and also another composite signal obtained from a second main channel signal constituted by a sum combination of third and fourth audio signals and a second sub-channel signal constituted by a frequency or phase modulation of a difference combination of said third and fourth audio signals on a predetermined subcarrier is recorded on another sound groove. In the reproduction of this record, the composite signals are first reproduced by a pick-up, and then the main channel signals and sub-channel signals are separated by filters or the like. Then, the sub-channel signals are detected, and the detection outputs and main channel signals are combined in matrices to derive separate front and back left and right signals. However, since the subchannel signals are frequency modulation or phase modulation signals of very high frequencies, it is likely that they will not be reproduced with fidelity and may be interrupted by such cause as the wear of the pick-up needle tip resulting in a loud foreign noise sound. In order to minimize the noise sound resulting from interruption of the sub-channel signals, it has been the practice to detect the interruption of the sub-channel signals and disconnect the sub-channel signal reproducing circuits thereby preventing them from producing an output during the absence of the subchannel signals. Even with such arrangement, however, a harsh foreign sound inevitably results at the time of the start or end of playback of the record because the subcarrier wave of the sub-channel signal is close to the highest frequency of the main channel signal and because it is necessary to use pulse counter type detectors. While the pick-up needle tip is scanning the soundless groove of the record, the output of the pickup is substantially Zero. Also, during this time the muting circuit is operative, so that no substantial noise sound is produced. However, when the pick-up needle tip reaches the sound groove to reproduce the sub-carrier waves of the sub-channel signals, the muting circuit is immediately rendered inoperative, and also at the same time the pick-up output is suddenly increased. Therefore, a large d-c level change results at the output side of the detector, and this change is passed through the muting circuit to the loudspeakers resulting in the production of a harsh foreign sound.

In another aspect, while this type of four-channel stereophonic reproducing system is generally capable of reproducing two-channel stereophonic records as well as four-channel records, if it is used for reproduction of a four-channel record with only two speakers (as in reproduction of a two-channel record) only front signals or back signals will be reproduced. This is a great inconvenience. Accordingly, for an audiophile who is inclined to first purchase only a four-channel stereophonic reproducing system and two loudspeakers and subsequently purchase the additional two loudspeakers when he has obtained the extra money required, a system is desired which permits reproduction of a fourchannel record as a two-channel reproduction of sufficient quality to provide good listening even with only two loudspeakers.

The present invention is directed to solution of the above in that and it provides a four-channel stereophonic reproducing system, in which generation of the afore-mentioned foreign sound is eliminated, and which provides a sufficiently listenable reproduction of a four-channel record as a two-channel reproduction. In order for the invention to be fully understood, it will now be described in conjunction with an embodiment thereof with reference to the accompanying drawing, in which:

FIG. 1 is a block diagram of a four-channel stereophonic reproducing system embodying the invention; and

FIG. 2 is a circuit diagram showing a specific circuit construction of part of the same embodiment.

Referring to the drawing, and particularly to FIG. 1, reference numeral 1 designates a pick-up, and numerals 2 and 3 respectively designate right channel and left channel signal processing sections. Although the left channel signal processing section 3 is not shown in detail, it has the same construction as the right channel signal processing section 2, and here the right channel signal processing section 2 will be typically described. Numeral 4 designates a pre-amplifier for amplifying the stereophonic composite signal detected by the pick-up 1, numeral 5 a low-pass filter for detecting only the main channel signal from the amplified stereophonic composite signal output of the pre-amplifier 4, numeral 6 a phase regulator for regulating the phase of the main channel signal detected by the low-pass filter 5, numeral 7 a separation regulator for regulating the separation by controlling the level of the phase controlled main channel signal output of the phase regulator 6, numeral 8 a matrix circuit, numeral 9 a sub-channel signal amplifier for selectively amplifying the subchannel signal contained in the amplified stereophonic composite signal output of the pre-amplifier 4, numeral 10 an amplitude limiter, numeral 11 a detector, numeral 12 a muting circuit, numeral 13 an FM/PM equalizer, numeral 14 a d-c amplifier for detecting the sub-carrier wave from the amplitude limited subchannel signal and producing a d-c output in proportion to the detected sub-carrier wave, numeral 15 a switching circuit onoff operated according to the output of the d-c amplifier 14, numeral 16 a switching circuit on-off operated according to whether the amplitude limited sub-channel signal is present or not, and numeral 17 a four-channel stereo indicator operated by the output of the switching circuit 16.

FIG. 2 shows the detailed circuit construction of the muting circuit 12, switching circuit 15 and fourchannel stereo indicator 17. As is shown, the muting circuit 12 consists of a field-effect transistor TR with the source and drain thereof respectively connected to the output terminal of the detector 11 and the input terminal of the equalizer 13. The gate of the transistor TR is connected through a diode D and a capacitor C to ground. The switching circuit 15 consists of a transistor TR with its base connected through a resistor R to the output terminal of the d-c amplifier 14, its emitter connected directly to ground, and its collector connected through a resistor R to the juncture between the resistor R and diode D. The four-channel stereo indicator 17 comprises a transistor TR which has its base connected to the output terminal of the switching circuit 16 and its emitter connected through a select switch SW to a power supply +B, and an indicator lamp PL connected through a resistor R to the collector of the transistor TR The juncture between the resistor R and indicator lamp FL is connected through a resistor R to the juncture between the diode D and capacitor C. Resistors R and R, serve as biasing resistors for the field-effect transistor TR a resistor R serves as a collector resistor for the transistor TR and a resistor R is provided between the juncture between resistor R and diode D and ground.

In operation, the stereophonic composite signal output produced from the pick-up l and coupled to each channel signal processing section is amplified through the pre-amplifier 4, whose output is coupled to the lowpass filter 5 and sub-channel signal amplifier 9 respectively separating the main channel signal and subchannel signal. The separated sub-channel signal goes to the amplitude limiter 10 for amplitude limitation and then to the detector 1 1 for detection before being coupled to the muting circuit 12. Assuming now that the sub-channel signal prevails with the normal sub-carrier wave, the d-c amplifier l4 and switching circuit 16 provide predetermined outputs, so that the transistor TR is off and the transistor TR is on. In this case, the power supply voltage +B is supplied, provided the pole of the select switch SW is thrown to the side of the fourchannel terminal a, through the resistor R to the indicator lamp PL, so that the indicator lamp FL is on. In this state, the capacitor C is charged due to the voltage drop across the indicator lamp PL. Meanwhile, since the transistor TR is off, its collector is at a sufficiently large positive potential, which is coupled to the gate of the field-effect transistor TR Thus, the transistor TR which constitutes the muting circuit 12 is on, and the detected signal from the detector 11 is transmitted through the muting circuit 12 and FM/PM equalizer 13 to the matrix circuit 8. In consequence, the signal passed through the equalizer 13 is combined with the main channel signal separated through the low-pass filter 5 and coupled through the phase regulator 6 and separation regulator 7, whereby predetermined front and back signals can be taken from the output terminals 18 and 19.

Now, if the sub-carrier wave of the sub-channel signal ceases due to such causes as wear of the pick-up needle tip etc., the output of the do amplifier 14 is momentarily extremely reduced, so that the transistor TR: is triggered, with its collector potential suddenly reduced to ground potential. As a result, the field-effect transistor TR is cut off, so that it no longer couples the output of the detector 11 to the equalizer 13. In consequence, the noise due to the cessation of the subcarrier wave will not be introduced as an output, and superior reproduction can be obtained.

The operator at the time of start of playback of the record will now be discussed. Before the start of playback of the record, at which time no stereophonic composite signal prevails, neither d-c amplifier 14 nor switching circuit 16 provide an output, and no charge is stored in the capacitor C. When playback of the record is started from this state, the subcarrier waves alone for the sub-channel signals are first reproduced. As a result, predetermined outputs appear from the d-c amplifier l4 and switching circuit 16, thus triggering the transistor TR and cutting off the transistor TR;,. From this moment, charging of the capacitor C begins. When a predetermined terminal voltage is built up across the capacitor C, the field-effect transistor TR is rendered on, whereupon the output of the detector 11 is coupled to the equalizer in the manner mentioned above in connection with the normal operation. The detector 11 becomes operative as soon as the subcarrier wave appears, and it suddenly produces a large d-c output from that moment. On the other hand, the muting circuit 12 is not renderedperfectly operative until the capacitor C is charged to a predetermined extent. This means that the d-c signal coupled through the muting circuit 12 to the equalizer 13 is gradually increased from the afore-said moment. Thus, with the above embodiment the harsh foreign sound that would be produced with the prior art system at that moment can be perfectly eliminated.

Also, by throwing the select switch SW to the side of the two-channel terminal b, two-channel reproduction of a four-channel record can be obtained. With the select switch SW thrown to the side of the two-channel terminal, the transistor TR is always off irrespective of the presence or absence of the subchannel signal, so that the indicator lamp PL remains off providing a comparatively low resistance. In consequence, the capacitor C can not be charged enough to let the fieldeffect transistor TR be rendered on upon appearance of the predetermined output from the d-c amplifier 14. This means that in this case passage of the sub-channel signal is completely blocked, and only the main channel signal is added to the matrix circuit 8, and it is taken out in situ from the output terminals 18 and 19.

As is apparent from the above embodiment, according to the invention a capacitor adapted to begin to be charged with the appearance of the subchannel signal is used to render inoperative the muting circuit by utilizing the terminal voltage across the capacitor, so that it is possible to eliminate harsh foreign sounds at the time of start of playback of a record. Also, since in the four-channel stereophonic reproducing system according to the invention the stereo indicator circuit, which is operated according to whether the sub-channel signal is present or not, is provided with a select switch capable of rendering the indicator inoperative irrespective of the presence or absence of the sub-channel signal. Accordingly, it is possible to reproduce a fourchannel record as a two-channel reproduction with sufficient fidelity the passage of the subchannel signals being cut by the muting circuit held inoperative by preventing charging of the capacitor and rendering the stereo indicator inoperative through the select switch. Thus, this system can be used even where only two loudspeakers are provided, which is very useful in practice.

What we claim is:

1. A four-channel stereophonic reproducing system for reproducing a CD4 type four-channel record where a composite signal obtained from a first main channel signal constituted by a sum combination of first and second audio signals and a first sub-channel signal constituted by a frequency or phase modulation of a difference combination of said first and second audio signals on a predetermined sub-carrier is recorded in one sound groove, and also where another composite signal obtained from a second main channel signal constituted by a sum combination of third and fourth audio signals and a second sub-channel signal constituted by a frequency or phase modulation of a difference combination of said third and fourth audio signals on a predetermined sub-carrier is recorded on another sound groove, said system comprising a pickup for reproducing the stereophonic composite signals from said four-channel record, filter means for separating the first and second main channel signals and first and second sub-channel signals from the reproduced stereophonic composite signals, first and second subchannel signal processing systems including pulse counter type detector means for detecting the separated first and second sub-channel signals respectively, matrix means for combining the outputs of said detector means and said first and second main channel signals to derive said first, second, third and fourth audio signals, muting circuit means each provided in each of the first and second sub-channel signal processing systems for on-off controlling the passage of each of the first and second sub-channel signals, and muting circuit control means for rendering operative said muting circuit means upon detection of the sub-carrier waves of said first and second sub-channel signals at the time of start of playback of said four-channel record and gradually rendering inoperative said muting circuit means after the detection of said sub-carrier waves.

2. The four-channel stereophonic reproducing system according to claim 1, wherein said muting circuit control means each have a parallel circuit of a resistor and a capacitor, the charging and discharging of said capacitor being utilized to on off control the associated muting circuit means.

3. The four-channel stereophonic reproducing system according to claim 1, which further comprises stereo indicator means operated according to whether said first and second sub-channel signals are present or not, and in which said muting circuit control means each have a switching circuit on-off controlled according to whether the sub-carrier wave of each of said first and second sub-channel signals is present or not and a capacitor adapted to be charged and discharged according to the output of the associated stereo indicator means, said capacitor being adapted to be charged at the time of start of playback of said four-channel signal, the terminal voltage built up at this time across said capacitor having the effect of gradually rendering inoperative the associated muting circuit means which is controlled by the output of the associated switching circuit and said terminal voltage across said capacitor.

4. The four-channel stereophonic reproducing system according to claim 3, wherein said stereo indicator means each include select switch means having a twochannel terminal and a four-channel terminal, said stereo indicator means being rendered inoperative irrespective of the presence or absence of the first and second sub-channel signals by throwing said select switch means to the two-channel terminal, whereupon the charge stored in each said capacitor is adapted to be reduced to render each said muting circuit means operative irrespective of the output of each said switching circuit, thereby opening the first and second subchannel signal processing circuits.

5. The four-channel stereophonic reproducing system according to claim 4, where said muting circuit means each comprises'a field-effect transistor having a source terminal to be used as a corresponding subchannel signal input terminal, a drain as its output terminal and a gate as control terminal connected through a resistor, a diode and a capacitor to ground, said switching circuits each comprise a transistor having its base connected to receive an output representing the presence or absence of the sub-channel wave of each of the first and second sub-channel signals, its collector connected through a resistor to the juncture between a resistor connected to the gate of the field-effect transistor and said diode and its emitter connected directly ground, and said stereo indicator means each comprise a second transistor having its base connected to receive an output representing the presence or absence of the first or second sub-channel signal and its emitter connected through the select switch to a power supply when said select switch is thrown to the the fourchannel terminal, and an indicator lamp connected through a resistor to the collector of said second transistor, a juncture between said indicator lamp and resistor being connected through a resistor to the juncture between said diode and capacitor. 

1. A four-channel stereophonic reproducing system for reproducing a CD-4 type four-channel record where a composite signal obtained from a first main channel signal constituted by a sum combination of first and second audio signals and a first sub-channel signal constituted by a frequency or phase modulation of a difference combination of said first and second audio signals on a predetermined sub-carrier is recorded in one sound groove, and also where another composite signal obtained from a second main channel signal constituted by a sUm combination of third and fourth audio signals and a second sub-channel signal constituted by a frequency or phase modulation of a difference combination of said third and fourth audio signals on a predetermined sub-carrier is recorded on another sound groove, said system comprising a pick-up for reproducing the stereophonic composite signals from said four-channel record, filter means for separating the first and second main channel signals and first and second sub-channel signals from the reproduced stereophonic composite signals, first and second sub-channel signal processing systems including pulse counter type detector means for detecting the separated first and second sub-channel signals respectively, matrix means for combining the outputs of said detector means and said first and second main channel signals to derive said first, second, third and fourth audio signals, muting circuit means each provided in each of the first and second sub-channel signal processing systems for on-off controlling the passage of each of the first and second sub-channel signals, and muting circuit control means for rendering operative said muting circuit means upon detection of the sub-carrier waves of said first and second sub-channel signals at the time of start of playback of said four-channel record and gradually rendering inoperative said muting circuit means after the detection of said sub-carrier waves.
 2. The four-channel stereophonic reproducing system according to claim 1, wherein said muting circuit control means each have a parallel circuit of a resistor and a capacitor, the charging and discharging of said capacitor being utilized to on-off control the associated muting circuit means.
 3. The four-channel stereophonic reproducing system according to claim 1, which further comprises stereo indicator means operated according to whether said first and second sub-channel signals are present or not, and in which said muting circuit control means each have a switching circuit on-off controlled according to whether the sub-carrier wave of each of said first and second sub-channel signals is present or not and a capacitor adapted to be charged and discharged according to the output of the associated stereo indicator means, said capacitor being adapted to be charged at the time of start of playback of said four-channel signal, the terminal voltage built up at this time across said capacitor having the effect of gradually rendering inoperative the associated muting circuit means which is controlled by the output of the associated switching circuit and said terminal voltage across said capacitor.
 4. The four-channel stereophonic reproducing system according to claim 3, wherein said stereo indicator means each include select switch means having a two-channel terminal and a four-channel terminal, said stereo indicator means being rendered inoperative irrespective of the presence or absence of the first and second sub-channel signals by throwing said select switch means to the two-channel terminal, whereupon the charge stored in each said capacitor is adapted to be reduced to render each said muting circuit means operative irrespective of the output of each said switching circuit, thereby opening the first and second sub-channel signal processing circuits.
 5. The four-channel stereophonic reproducing system according to claim 4, where said muting circuit means each comprises a field-effect transistor having a source terminal to be used as a corresponding sub-channel signal input terminal, a drain as its output terminal and a gate as control terminal connected through a resistor, a diode and a capacitor to ground, said switching circuits each comprise a transistor having its base connected to receive an output representing the presence or absence of the sub-channel wave of each of the first and second sub-channel signals, its collector connected through a resistor to the juncture between a resistor connected to the gate of the field-effect transistor and said diode and its emitter connEcted directly ground, and said stereo indicator means each comprise a second transistor having its base connected to receive an output representing the presence or absence of the first or second sub-channel signal and its emitter connected through the select switch to a power supply when said select switch is thrown to the the four-channel terminal, and an indicator lamp connected through a resistor to the collector of said second transistor, a juncture between said indicator lamp and resistor being connected through a resistor to the juncture between said diode and capacitor. 